Bacterivory by mixotrophic flagellates may contribute to their nutrient supply, providing a competitive advantage in oligotrophic waters. We hypothesized an increase in Dinobryon biomass during the re-oligotrophication process in the large and deep Lake Constance. To estimate whether bacterivory contributed substantially to the flagellates’ phosphorus supply, we determined ingestion rates. Dinobryon biomass increased with decreasing total phosphorus concentrations in the lake over a period of 17 years (P = 0.0005). The promotion of Dinobryon biomass during re-oligotrophication may be explained by the increasing light availability due to the decreasing biomass of other phytoplankton yielding a release from competition. The date of the Dinobryon abundance maximum shifted to earlier time points in the year, probably because a smaller phosphorus pool was depleted more quickly. Ingestion rates of Dinobryon ranged between 0.5 and 13 bacteria cell⁻¹ h⁻¹ (0.2–5.4 fg C pg C⁻¹ h⁻¹), and clearance rates varied between 0.2 and 3.2 nL cell⁻¹ h⁻¹ (4–78 pL pg C⁻¹ h⁻¹), leading to bacterial losses of up to 30% day⁻¹ of bacterial standing stock. The ingestion of bacteria covered 77% of the phosphorus need of the flagellate during the period of maximum growth in 1996 (net growth rate 0.34 day⁻¹), and it fully covered the need at all other times.